It's about 150 times the number of Union Stations in the United States.

In other words, 18,000 is 133 times the count of Union Stations in the United States, and the count of Union Stations in the United States is 0.00752 times that amount.

(a.k.a. Union Terminal; a.k.a. Joint Station; a.k.a. Union Depot) (2016 figures)Typically shared between multiple railroad companies and stations, there are about 135 "Union Stations" in the United States. Of these, the busiest station is Chicago Union Station, which handles 120,000 passengers each year.

It's about 150 times the number of Wolves in Yellowstone National Park.

In other words, 18,000 is 173 times the count of Wolves in Yellowstone National Park, and the count of Wolves in Yellowstone National Park is 0.00578 times that amount.

(a.k.a. Henihco'oo; a.k.a. Héetíhco'oo) (December 2014 figures; Wyoming Park area)
As of the 2014 population count, there were 104 wolves living in Yellowstone National Park, in twelve packs. All of the wolves now living in the Park are thought to have descended from the 66 wolves (re)introduced into the Park in 1995-96.